1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and a developing device for the same.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a latent image on a photoconductive drum or similar image carrier, develop the latent image with a developing device, which stores a developer therein, and then transfer the resulting toner image to a sheet-like recording medium. The developer is, in many cases, implemented as a two-component type developer consisting of toner grains and magnetic carrier grains because this type of developer is feasible for color image formation.
The developer is agitated and mixed in the developing device to be charged by friction. The toner grains electrostatically deposit on the carrier grains thus charged. The carrier grains, holding the toner grains thereon, are deposited on a sleeve or tubular developer carrier by being attracted by a magnet disposed in the sleeve. The sleeve in rotation conveys the developer deposited thereon to a developing zone.
A main magnet for development is disposed in the sleeve at, in a facing zone where the drum and sleeve face each other, the closest position. As the developer on the sleeve approaches the main magnet, a number of carrier grains in the developer gather and rise in the form of brush chains along the magnetic lines of force issuing from the main magnet, forming a magnet brush.
As for development using the magnet brush mentioned above, the magnet brush contacts the drum in a developing zone. In this condition, the carrier grains, which are dielectric, are presumed to intensify an electric field between the drum and the sleeve for thereby causing the toner grains to fly from the carrier grains present on the tips of the brush chains directly to a latent image or drum surface. According to this presumption, however, the conventional magnet brush type development has a problem that development is effected only by the toner grains transferred from the brush chains to a latent image in a limited portion around the closest position. Stated another way, in a portion where the magnet brush is absent and a portion where it does not contact a latent image, development by the toner grains directly transferred from the tips of the magnet brush to the latent image does not occur at all. More specifically, the toner grains can develop a latent image only in a limited portion where the tips of the magnet brush contact the drum. It is therefore extremely difficult to increase the number of toner grains contributing to development by controlling conditions other than the above limited portion.
To implement a high-density image in such a limited portion, Japanese Patent No. 2668781, for example, discloses a developing method that uses both of toner grains deposited on the brush chains of carrier grains and toner grains deposited on the sleeve for development by using an alternating electric field. This developing method, however, has some problems left unsolved. First, a developing zone available is only the portion where the carrier grains contact the drum, so that sufficiently high image density is not easy to attain only with the toner grains held on the carrier grains and toner grains present on the sleeve in the above developing zone. Second, the number of brush chains is too small to realize a smooth, high quality solid image with an electrode effect. Third, the electric field causes the toner grains deposited on, e.g., the brush chains to move toward the sleeve, smearing the sleeve. This makes the electric field different from the surrounding and therefore causes a residual image to appear in a halftone image.
Japanese Patent Laid-Open Publication Nos. 6-208304 and 7-319174, for example, each propose to deposit toner on a photoconductive element and then remove excess part of the toner for thereby implementing high image density and reducing fog. For this purpose, magnetic toner deposited on the surface of an image carrier, accommodating a magnet therein, is brought into contact with an electrode roller, also accommodating a magnet therein, so that unnecessary toner is removed from portions other than an image portion. Further, Japanese Patent Laid-Open Publication No. 5-46014 proposes to effect development with a first developing roller and then remove excess toner with a second developing roller to which only a carrier is fed.
However, a problem with technologies taught in Laid-Open Publication Nos. 6-208304 and 7-319174 mentioned above is that a magnet must be disposed in the photoconductive element as well. This, coupled with the fact that such technologies are applicable only to a developing system using magnetic toner, increases cost and cannot meet the demand for color image formation. The scheme of Laid-Open Publication No. 5-46014 is not practicable without resorting to two developing rollers and without constantly feeding only fresh magnetic carrier, resulting an increase in cost.
On the other hand, Japanese Patent Laid-Open Publication No. 9-222799 pertains to the flight of toner grains and teaches a relation between the configuration effect of one-component toner having a grain size as small as 4 μm or less and air resistance specifically.
In anyone of the conventional schemes described above, only the region where the magnetic grains rub the photoconductive drum constitutes a developing zone. It is therefore difficult to achieve sufficiently high image density with only toner grains deposited on brush chains present in the developing zone and toner grains deposited on the drum. Further, because the number of brush chains is small, it is difficult to implement a smooth solid image with an electrode effect. Moreover, it is difficult to obviate background contamination by controlling the deposition of toner grains in portions other than an image portion.
Japanese Patent Laid-Open Publication No. 5-303284, for example, discloses a non-contact type developing system in which two magnetic poles are positioned at both sides of a developing zone close to an image carrier while the distance between image carrier and a developing sleeve is selected to be greater than the thickness of a developer layer formed on the developing sleeve. In this condition, the developer is caused to jump for effecting development. Although this developing system is capable of desirably reproducing a highlight portion and implementing a faithful halftone portion, it sometimes renders a black solid image short in density or blurred due to short developing efficiency, as determined by experiments. It is therefore necessary to further improve image quality as to developing efficiency and the density of a black solid image.
Another developing method, which is new but not laid open to public inspection yet, uses free toner grains for development. More specifically, a developer carrier, accommodating a magnet therein, faces an image carrier and conveys a toner and carrier mixture forming a layer thereon. A difference in speed is established between the developer carrier and the magnet in order to cause the developer layer to flow while forming a magnet brush at least in a zone where the developer carrier and image carrier face each other. While the developer carrier is flowing, free toner grains part from magnetic carrier grains and deposit on a latent image formed on the image carrier. Because the free toner grains contribute to development, the developing zone available with this developing method is broader than the developing zone of the conventional magnet brush type developing method that causes magnetic carrier grains to directly contact the image carrier, as will be described more specifically later. This successfully increases the amount of development and therefore enhances developing efficiency for thereby realizing a high-density solid image, as determined by experiments.